Due to their rapid curability, for radiation-curable compositions, energy and time required for drying can be much reduced as compared with the conventional solvent type resin compositions, and, in addition, drying apparatuses are not needed. Thus, saving of spaces can be attained. Furthermore, the compositions require only a small amount of solvents or require no solvents. For these reasons, they are used in a yearly increased amount as materials safe for earth environment.
Moreover, recently, the radiation-curable compositions expand in their use to various fields, but performances required in these fields sometimes cannot be attained by only the combination of oligomers or monomers conventionally used as starting materials.
On the other hand, molded articles made from polymethyl methacrylate resins, polycarbonate resins, etc. have various advantages such as light weight and excellent shock resistance and, easy processing, and are used in many fields. However, since these plastic molded articles are insufficient in abrasion resistance of the surface, their surface is apt to be damaged and improvement in abrasion resistance is demanded. Furthermore, these molded articles are sometimes used outdoors such as automobile parts, and are also strongly demanded to have weather resistance.
In order to improve abrasion resistance, methods of coating the surface of these plastic molded articles with ultraviolet-curable compositions have been investigated. However, these compositions are sometimes insufficient in abrasion resistance and adhesiveness to plastics, and even if these performances are satisfied to some extent, in many cases, there are problems in weather resistance.
That is, in the case of ultraviolet-curable compositions mainly composed of acrylates, which are in most cases used as radiation-curable compositions, photopolymerization initiators that generate active radicals by the irradiation with ultraviolet rays must be added to the compositions for curing them with ultraviolet rays. However, the photopolymerization initiators remain in cured products of the compositions and deteriorate weather resistance of the cured products to cause coloration or discoloration, peeling of coat and cracking. Therefore, the ultraviolet-curable compositions are unsatisfactory for the use requiring weather resistance. Further, decomposition products of photopolymerization initiators in the cured products sometimes give odor to the cured products.
Furthermore, it has been attempted to improve weather resistance of the compositions by adding weather resistance improvers such as ultraviolet absorbers, light stabilizers and antioxidants, but the effect is still insufficient and, besides, there are problems that the weather resistance improvers hinder the curing reaction to cause deterioration in ultraviolet-curability of the compositions and result in reduction of productivity.
Recently, it has been found that N-substituted maleimide compounds have a function as photopolymerization initiators, and it has been reported that vinyl ether or an acrylate is polymerized by ultraviolet rays without using photopolymerization initiators [Sonny Jonsson et al, Radotech 95 Europe, Previous Lecture Manuscripts "Academic Day", page 34].
This function of N-substituted maleimide compounds to initiate photopolymerization is excellent, being different from conventional photopolymerization initiators, but since these maleimide compounds are solid and high in melting point, they are difficult to handle, and, furthermore, they must be dissolved in acrylates in order to use them in the form of liquid. However, maleimide compounds are sometimes low in solubility in acrylates, and, in this case, if the amount of maleimide compounds is increased, they are precipitated. Due to these problems, only curable compositions of limited formulations can be produced, and when formulations of compositions are changed depending on the properties demanded in various uses, the compositions do not satisfy the desired properties.
In addition, these maleimide compounds are low-molecular weight compounds, and when they are added to curable compositions as a component having a function as photopolymerization initiators, if they remain without being bonded to the cured products, characteristics of the cured products are deteriorated.
Furthermore, these maleimide compounds are generally produced by addition reaction of maleic anhydride with amines and the subsequent dehydration reaction, but this method produces them in low yields owing to side reaction of unsaturated group of the starting maleic anhydride. A method of production with protecting the unsaturated group was proposed (JP-A-2-268155), but a step of deprotection reaction is added and the production is not simple.
Based on the above facts, the inventors have conducted an intensive research in an attempt to find a curable composition which is easy in preparation of its starting materials, is easily curable by irradiation with radiations, especially ultraviolet rays, provides cured products excellent in weather resistance and abrasion resistance, has no problem of odor, and is excellent in adhesion to substrate.